CA2005553A1 - Method for acoustically measuring wall thickness of tubular goods - Google Patents

Abstract

METHOD FOR ACOUSTICALLY MEASURING
WALL THICKNESS OF TUBULAR GOODS

Abstract of the Disclosure A method for acoustically measuring wall thickness of tubular goods utilizes an acoustical transducer with a large bandwidth and high resonant frequency to measure the thickness of thin walls. The transducer is exposed directly to fluids in the tubular goods and is rotatable 360 degrees to scan the entire circumference of the tubular goods. The transducer is excited and generates an acoustic waveform from the wall under investigation.
The acoustic return has a reverberation portion which results from the reverberation of the generated acoustic waveform in the wall. The harmonic frequency content of the reverberation portion is determined and the frequency difference between two adjacent harmonics is determined, from which the wall thickness is determined.

Description

200~5~;~

Docket No. DF-679 METHOD FOR ACOUS~ICALLY MEASURING
WALL THICXNESS OF TUBULAR GOQ~S

S~ecification Field of the Invention The present invention relate~ to methods for acoustically measuring wall thickness of tubular goods such as well casing, production tubing, pipelines and the like, wherein the presence of corrosion on the tubular good~ can be determinQd.

~ack~round of the Inv~ntlQn In oil and natural gas wells, casing is set into the well borehole. In the borehole, the casing is sub~-ctod to corrosive elements which act on the casing by reducing the casing wall th~ckness. Cement, which has been ~orced into the annulus between the casing and the surrounding ~trata, protects the outer surface of the cas~ng from corro6ion. But occasionally the bond between the cement and the casing is unsatisfactory, wherein the outer surface of the casing i6 exposed to corrosive elements. Furthermore, casing is typically cemented only for portions of its length, leaving exposed portions.
It is desirable to periodically sur~ey the casing to determine the pre~ence of corrosion and inside æurface wear on the casing and the extent of damage.
Corrosion (and wear) reduces the wall thickness of the caS1ng, ~o by mea~uring the wall thickness, the presence or absence of corrosion can be inferred. once corrosion has been detected on the casing, the situation can be evaluated and ~ decislon can be made on whether or not to take corrective action, which corrective action is time consuming and expensive. In order to properly evaluate whether to t~ke corrective action, the wall thicknes~ o~ the corroded area, the extent o~ corrosion, and the location o~ the corrosion are ~actors which are con~idered.
In the prior art, there are apparatuses and methods for ultra~onically scanning ca~ing to determine the presonce of corro~ion. ~owever, the prior art fails to provide enouqh information on corrosion to make a proper ~valuation. The prior art i~ exempli~ied by Zemane~, which U.S. Pat. No. 3,369,626 discloses an ultrasonic apparatu~ ~or u~e in scanning the inner sur~ace of ca~ing on the inner ~ur~ace of an open well borehole.
The ultra~on~c apparatu~, which i8 commercially ~nown as the borehol~ tol-viewer, use~ a rotatinq ultrasonic tran~duc-r hou~ed inside of an acoustical window to provide ~ull coverage of the casing, thus providing high areal re~olution. The borehole televiewer is limited, however, to canning the inner ~ur~ace o~ casing, and thu~ cannot be u~ed to determine corrosion occurring on the outer ~urfac~ o~ the ca~ing. Furthermore, the borehole televiewer uses analog signal proce~sing, thereby limiting the capab$1ities of the apparatus.
Havira, U.S. Pat. No. 4,255,798 discloses an ultrasonic logging apparatus for evaluating the bond ~etween the cement and the casing. The apparatus also datermines the wall thickness of the casing ~or corrosion detection purposes. One ver~ion of the lo~ging apparatu~ utilizes a ~ingle fixed transducer ~nd a rotating reflector. The transducer has a bandwidth of 300 KHz - 600 KHz, in order to excite the fundamental re~on nce of the casing wall and determine wall thickness. The relatively low bandwidth limits the resolution of the logqing apparatus in determining casinq wall thic~nes~. Wall~ thinner than about Smm are ` 2005553 unresolvable. It is frequently desired to resolve thinner walls. Furthermore, the presence of the acoustical window in the path of the acoustic return cau~es distortion of the acoustic return, further limiting the resolution. The acoustical window could be impedance matched to reduce distortion, however varying condltion~ downhole, including mud waights and temperatures, make impedance matching o~ the acoustic return i~practical.
Another vorsion of the logging apparatus of Havira, U.S. Pat. No. 4,255,798 uses transducers which are directly oxposed to the borehole fluids. Plural tran~ducers are provided in a circumferential arrangement. Becaus~ transducers are fixed full coverage of tho casing wall is not provided for, thus limitlng th- areal r~solution of the logging apparatus.
What i~ needed is a method that measures wall thicknes~, whlch method has high cas~ng wall thickness resolution and that provide~ full areal resolution of the casinq wall, wherein corro~ion on the casing wall can be detected.

Summary of the Invent~on It is an ob~ect of the present invention to provid~
a method for ultrasonically measuring wall thickness of tubular goods so that the presence of corrosion can be determined, which method has high cas~ng wall thickness resolution ~nd provides full areal resolution of the tubular goods.
The method of the present invention provides an acou~tical transducer means inside of the tubular goods which are to be ~nvest~gated. ~he transducer means has a resonant freguency greater than 1MH2 and is directly exposed to fluids in the investigated tubular goods.
The transducer means i8 rotatable so as to scan 360 degree~ o~ the circu~ference of the invsstigated tubular goods. The transducer means i~ excited to generate an acoustic waveform, which i~ directed to a portion of the investigated tubular good~. The transducer means then receives the re~ulting acoustic return. The acoustic return include~ a reverberation portion which occurs as a re~ult of the generated acoustic waveform rev~rberating inside of the investigated tubular goods portion. The reverberation portion i~ selected from the acoustic r~turn, and th~ harmonic frequency content of th~ reverberation portion i8 determined. The frequency dlfference between two ad~acent harmonic freguencies is determined and the wall thickne~ o~ the investigated portion i~ dotQrained froa th- ~reguency differences.
i Brie~ Description of the Drawinas Flg. 1 i~ a ~chematic longitudinal cross-sectional view o~ a cased well borehole, showing an ultrasonic logging apparatu~ therein, and supporting surface equipm~nt, with which the method of the present in~ention, in accordance with a pr~ferred embodiment, can be practiced.
Fig. 2 is a detail view of the transducer portion of the logging apparatus of Fig. 1.
Fig. 3 i~ a block diagram of the downhole electronic~ unit which is located in the ultrasonic logging apparatu~.
Fig. 4 i~ a block diagram of the equipment which is u~ed ~o process the data obtained fro~ the ultrasonic logging apparatu~ and the supporting urface equipment.
Fig. 5 is a tran~versQ cross-sectional view of a cased borehole, with a diagrammatic representation of the acoustic reflect;on~ and reverb~rations caused by the casing.
Fiq. 6 i~ a graph showing an acoustic return of a ca~ing wall.

200555~

.

Fig. 7 is the ~requency spectrum of the reverb~ration portion of the acoustic return of Fig. 6.
Fig. 8 ~ an autocorrelated frequency spectrum of the spectrum of Fig. 7.
Fig. 9 i~ a graph illustrating the determination of the confidence level of a ~chematic autocorrelated reverberation portion.
Fig. 10 i8 a graph showing ultrasonic measurements of a tran~verse section of casing, utilizing the method of the present invention, which casing has a groove formed in its northwest circumferential portion.
Fig. 11 iB a graph ~howing ultrasonic measurements of a tr~nsverse section of casing, utilizing the method o~ the pre-ent invention, which casing has a flattened area in it~ northwe~t circumferential portion.
~ ig. 12 i~ a graph ~howing a generated acoustic wavefora as produc~d by the transducer.
Fig. 13 i~ an ultrasonic log of a portion of casing wall, ~hown unwrapped.

DescriDtion o~ Preferred Embodimen~
In Figs. 1 and 2, there i~ shown a schematic longitudinal cros~-sectional view of a cased well boreholo 11, showing an ultrasonic logging apparatus 13 located therein, and supporting surface equipment lS, with which the method of the present invention, in accordance with a preferred em~odiment, can be practiced.
The well borehole 11, which is drilled into the earth 17, ~s ~or producing oil or natural gas. The well borehole 11 is lined with a length of casing 19. The casing wall has inner and outer surfaces 21, 23. Cement 25 fill~ the annulus between the casing 19 and the walls o~ the borehole 11, for at least some of the len~th of the casing. The cement 25 is used to isolate one for~ation ~ro~ another and to support the casin~. The interior of the easing is filled with borehole fluids 27, which inelude drilling mud, oil, or both. The ea~ing has eorrosion 28A on its outer surfaee 23 and wear 28B on it~ inner surfaee 21. The easing may also have eorro~ion on its inner surfaee.
Both eorro~ion 28A ~nd wear 28B reduce the wall thiekness o~ the easing 19. The logging apparatus 13 and the method of the pre~ent invention measure wall thiekne~s, from whieh the pre~enee of eorros~on or wear ean be inferred.
The loqging apparatu~ 13 i8 loeated within the easing 19 and moves up or down th~ borehole for logging operation~. ThQ logging apparatus 13 is suspended inside ot the ea~ing by a logging eable 29, whieh provide~ eleetrieal power and eommunication channel~
from the surraee eguipment 15. The logging apparatu3 13 ineludes a transdueer portion 31, a motor portion 33, and an eleetronics portion 35. The tran~ducer portion 31 has an aeoustieal transdueer 37 mounted therein. The transdueer 37 i~ mounted ~o a~ to be direetly exposed to the borehole ~luids 27. The transdueer 37 is oriented so as to generate aeoustie waveforms which are normal to the walls oi the casing 19. The motor portion 33 of the logging apparatu~ prov~des the mechanical means to rotate the transducer portion 31 360 degrees within the ea~ing. U.S. patent application Serial No. 162,771, filed Marcb 1, 1988, which is owned by the assiqnee of the pr¢sent invention, shows and describes a logqing apparatu~ having a rotatable transducer portion and an exposed transducer, which i8 es~entially identical to the logging apparatus shown and described herein. The disclosure of U.S. patent application Serial No.
162,771, filed March 1, 1988 is incorporated by reference into this disclosure. The transducer 37 can scan the entire circum~erence of the casing wall. The loqqing apparatus is centered along the longitudinal axis of the casing by centralizers 39.
In the preferred embodiment, the acoustical transducer 37 has a resonant frequency of about 2MHZ and a bandwidth of about 1.0-2.5M~z. The transducer has a Z2~-8~ lead metaniobate ferroeloctric element with a front-2-2r~ ace, quarter-wave polymer matching layer cy~y~ and a low impedance, high attenuation bacXing layer. The transducer focal length can be modified to suit the casing diameter or the borehole diameter.
Referring to Fig. 3 ~ the electronics portion 35 of the logging apparatus 13 contains the downhole electronics, which inter~aces with the transducer 37 and p-rforms so~e preliminary processing of the data before tran~mitting th- data over the logging cable 29. The el-ctronics portion 35 include~ a digital signal proces~or 41, a digital-to-~naloq (D/A) converter and driver 43r a receiver 45, an analog-to-digital (A/D) converter 47, a logging cable interface 49, and a m~gneto~eter Sl. In the preferred embodiment, the digital signal processor 41 iB a TMS320C25 CMOS
(complimentary metal oxide semiconductor) integrated ci~cuit, manufactured by Texas Instruments. The digital ~ignal processor contains some memory. The digital ~ignal processor 41 is connected to a transmitter memory 53, a receiver me~ory 55, and the logging cable interface 49 by way of a data bus 57.
The transducer 37 generates an acoustic waveform which i~ directed to the casing wall 19. The interaction of the generated acoustic waveform with the casing wall produce~ an acoustic return. Referring to Fig~. 5 and 6, the acoustic return is made up of a reflection portion Eo~ where the generated acoustic waveform is reflected off of the inner surface 21 of the casing wall, and a reverberation portion E1,E2,E3 ....
where the gener~ted acoustic waveform reverberates .
.

inside of the casing wall between the inner and outer surfaces 21,23. Because of the high frequency o~ the generated acoustic waveform, for uncorroded casing, the reverberation portion of the acoustic return will typically contain only the relatively higher harmonics (e.g. 3rd or 4th harmonic) of the resonant frequency of th- ca~ing.
Ihe acoustic return i8 received by the transducer 37. It i8 desirable to obtain an acoustic return where the interference betwe0n the reflection portion and the reverberation portion i~ minimlzed, in order to simplify proce~sing and interpretation. The acoustic return can be manipulated, to a certain extent, by manipulating the qenerated acou~tic waveform. By producing a sharp qenerated acoustic wav-form from the transducer, the r-~ulting acou~tic return i~ not burdened with undue inter~erence. Th- de~irable generated acoustic waveform varies from tran~ducer to transducer and may vary under difter-nt borehole condition~.
In order to generate an acoustic waveform which re~ult~ in a "clean" acoustic return, regardless o~ the downhole conditions of the transducer type, it is desirable to produce an arbitrary acoustic waveform ~rom the transducer. The transmitter circuitry, which includes the digital signal processor 41, the trans~itter memory 53, the D~A converter and driver 43, and the transducer 37, can produce an arbitrary or progra~med waveform. The digital signa} processor 41 loads the transmitter memory 53 w~th the progra~med wav~for~ by way of th~ data bus 57. The programmed wave~or~ can either be resident in downhole memory accessible by the digital signal processor, or can be trans~itted fr~m the surface via the logging cable 29 and logging cable interface 49. The transmitter memory 53 i~ a first-in-fir~t-out (FIF0) memory device that outputs the digital waveform data to the D/A converter ZOO~S53 .

and driver 43. The D/A converter and driver 43 converts the digital waveform into an analog waveform and ampli~ies the wave~orm. The amplified waveform is sent to the transducer 37 where an acoustical waveform is generated.
In the preferred embodiment, the programmed waveform used to excite the transducer i8 determined by inv~rse ~ilter techniques such as are described in Schmolke et al., "Generation Of Optimal Input Signals For Ultrasound Pulse-Echo Systems~, 1982 Ultrasonics Symposium, IE~E, pages 929 st seq.
The receiver 45 receives the acoustic returns ~rom the transducer 37. The receiver filters and amplifies the acoustic return. The receiver 45 includes circuitry ~or protoctinq it~ ampli~ier from the transmitted wave~or~ sent to the transducer ~rom the D/A converter and drlver 43. Tbe acoustic return is sent from the r~ceiver 45 to th~ A/D converter 47 where the signal is digitized. The digitized acoustic return is loaded into the receiver memory 55, which is a FIFO memory unit.
ThQ digitized acoustic return is then either processed by the digital signal processor 41 or sent uphole by the logging cable inter~ace 4g.
The logging cable interface 49 ~oth transmits data uphol~ over the logging cable 29 and receives data sent downhole on the logging cable. Because of bandwidth limitations of the logging cable 29, the logging cable ~nterface 49 ~ay convert the digitized acoustic return into a form more suitable for transmission over the logging ca~le. In the preferred embodiment, the logginq cable interface includes a DJA converter for converting the digitized acoustic return into an analog signal, wherein the analog acoustic return is transmitted up the logging cable~ The digitized acoustic return is converted into an analog signal in order to ~ore effectively transmit data over the logging cable, which has a lia~ted bandwidth.
The magnetometer S1 provides information on the àzimuthal orientation of ths transducer 37 inside of the borehol- 11. The magneto~eter 51 i~ connected to the logging cable interface 49 in order to transmit its asimuthal information uphole over the logging cable.
~ eferring to Fig. l, the sur~ace equipment 15 will now be described. The ~urface equip~ent includes a logging cable interface 59, similar to the downhole logglng cable interface 49. Th- data received from the loqqing apparatus 13 i8 sent to a receiver 61 from the logging cable int-rface 59, where it is filtered and ~pl~fied. The r-ceiver 61 then ~ends the data to the analog ma~s storagQ unit 63, where the data is stored to await ~ubseguent proce~sing. ~he analog mass storage unit 63 can be, ~or example, a tape unit. The receiver 61, al~o ~ends the dat~ to an amplitude sample and hold unit 65, ~o that the amplitude of the acoustic return can be aonitored on the display 67 by an operator. The display 67 can also display an entire acoustic return.
Conventional depth instrumentation 68 provldes information on the depth of the logging apparatus. The operator can, through the operator interface 69, co~nunicate w~th the logging apparatus 1~ and the uphole receiver 61 to change various parameters of the equip~ent, such as the pul3e repetition rate of the transducer and tbe cable driving rate.
To log the casing l9 in the borehole 11, the logging apparatus 13 is lowered down into the borehole until thQ desired depth is reached (see Figs. 1 and 2).
The transducer portion 31 of the logging apparatus is rotated ~o that the entire circumference of the borehole i5 exposed to the transducer 37. As the transducer portion is rotated, the transducer 37 is periodically excited to produce a generat~d acoustic waveform (see Fig. 12). Then, the transducer receives the acoustic return fro~ the casing wall. As the transdueer portion is rotated, the logging apparatus 13 is pulled up the borehole. Thus, the logging apparatus 13 scans the casing wall in a helical pattern, with the individual seanned spots overlapping to provide full coverage. As a typical example, for most pipe sizes, if the transducer pulse repetition rate i8 200 pulses (or generated acoustie waveforms) per rotation, the transducer portion is rotated at 3 revolutions per seeond, the vertieal logging speed is 5 feet per second, and the tran~dueer spot size is 1/2 inehes, full eoverage of the easing wall can be obtained.
The ~-thod o~ proeesJing the data Yrom the aeoustie r~turns to determine easing wall thieXness will now be described. Beeause o~ the large amount o~ data generat-d by the logging apparatus, in the preferred embodi~ent, the data is stored on an analog mas~ 6torage unit 63 for ~ub-equent proee~sing. Fig. 4 illustrates the d~ta proeessing equip~ent, which lncludes the analog ma~s storage unit 63, a digitizer and receiver unit 71, a eo~puter 73, and a di~play unit 75.
To proeess the aeou~tie return to determine the per~odicity of the ¢a~ing rever~erations, the acoustic return is ~irst read ~rom the mass storage unit 63 to the digitizer and reeeiver 71. The digitizer and receiver 71 digit~zes, ~lters, and amplifies the reverberation portion o~ the acoustic return to form a t~me serie~. The digitizing rate is sufficiently fast so a~ to avoid any ali~sing of the signal. For a transducer having a bandwidth up to 3MHz, a sampling rate of lOMHz i5 used to avoid aliasing. The reflection portion need not be digitized and processed to determine wall thickness; although the reflection portion can be processed to investigate the ~nner surface of the casing a~ provided ~n Ze~anek, U.S. Pat. No. 3,369,626. The ~OOSS53 . .

digitized acoustic return is sent to the computer 73 for further processing.
To determine the thickness of the investigated ca~ing wall portion, the periodicity of the casing reverberation~ o~ the acoustic return is determined.
Referrlng to Fiq. 6, the reverberation portion time ~eries, which includes wavelet~ El,E2,E~ is passed through a weighted window 77 before the freguency spectrum o~ the reverberation portion is deter~ined.
The reverberation portion El,E2,E3... is multiplied by the window 77. The weighted window 77, with the shape and position relative to the acoustic return as shown in Fig. 6, is used to compensate for taking the Fourier tran~orm of a finite time series. The window 77 $s weighted by tapering it, with more weight going to the central portion (in Fig. 6 about 67 microseconds) of the time s-ries than to the end portions (about 61 microQeconds and 73 microseconds) of the time series.
Tapering reduce~ the side lobe~ in the ~reguency spectrum o~ the time series. In deter~ininq the proper weighting that i8 applied to the time series, there is a compromi~e that can be achieved bstween too little or too much ~eighting. A rectangular window (having equal w~ight across the time serie~) has a first side lobe that i8 too large. On the other hand, excessive tapering of the weighted window will reduce spectral resolution. A weightinq function that has been found to work satisfactorily is:

W(t~ Io(btl-t2/T2~l/2) Io(b) where Io i~ the modified Bessel function of the first kind of the order zero, t i~ the time sample in seconds, T is the window half-width in seconds, and b is a 200~5~i3 ~electable parameter. Values of b between 3 and 4 have been found to work well.
The next step involves taking the fast Fourier trans~orm of the windowed time series, in order to obtain th~ frequency spectrum of the time series. The magnitude spectru~ (see Fig. 7) is determined from the frequency spectrum by squaring the complex ti~e series o~ the ~requency ~pectrum and then determining the squar~ root of the real portion of the ~quared complex tire series. Alternatively, the power spectrum could be used, which i8 deter~ined from the real portion of the squared complex time ~eries. The magnitude spectrum is filtered by excluding time sa~ples which lie outside of the transducer bAndwidth (about l.OMHz to 2.7MHz in Fig.
7~.
Once the ~requency (magnitude) spectrum has been determined, the casing wall thickne~s at the investigated portion can be determined by looking at the harmonic frequencies present in the reverberation portion o~ the frequency spectrum. As a condition for resonance, which causes reverberation, there are an integral number of wavelength4 in twice the distance between the inner and outer surfaces 21,23 of the casing wall:

N ~= 2L

where N is an integer, ~ is the wavelength of the resonsnt acoustic signal, and L i8 the distance between the inner and outer surfaces of the casing wall.
~ _ V~f where f is the harmonic frequency and V is the velocity of xound in steel (6 m~microsecond).

-For the Nth harmonic:

~y = 2L
fN

so fN ~

For the Nth I 1 harmonic:

fN+1 ~ (N+l)v ~o th~ difference in harmonic frequencies is:
~N+l ~ ~N ' V

V
2(fN+l fNI

To determ~ne the wall thickness, the difference in harmonic freguencies is determined. In Fig. 7, the harmonic frequencies are shown by peaks. To determine the ~r~quency difference between peaks, that portion of the fre~uency spectrum which i8 within the transducer bandwidth (about 0.6 to 3MHz in Fig. 7) is auto correlated (see Fig. 8). The autocorrelation is deter~ined according to :
N-LAG
AC(LAG) ~ I ~ S(I)S(I~LAG) where AC is the autocorrelation, N is the number of points in the transducer bandwidth, S is the frequency spectrum ~as shown in Fig. 7), and LAG assumes values from O to N-l. The autocorrelation is searched for the position of a ~irst peak 79 which is the resonant ~requency of the casing wall portion (which in Fig. 8 is at about O.37MHz). Quadratic interpolation is used to determine the position and frequency Fp of the first peak. The wall thickness of the casing wall portion is then determined by using the frequency Fp in the eXpression fN+l - fN-An alternative method for f$nding the frequencydif~erence between harmonic frequencies can be used in lieu of autocorrelation. The digitized acoustic return time series is first squared. Then, the frequency gpectrum i8 determined from the sguared time series.
Before squaring the time series, the time series is filtered to reduce noise.
A confidence level can be calculated to measure the reliability of the wall thickness measurement.
Referring to Fig. 9, where an autocorrelation is shown schematically, the confidence level C is determined from:
C - AB
where A i~ the distance from the autocorrelation to a line connecting the initial peak at the origin to the fir~t peak at Fp, at a frequency midway between the origin and Fp, and B i8 the height of the f~rst autocorrelation peak. The confidence level provides an indication of the peakedness of the first autocorrelation peak. The higher the confidence level, the more reliable the wall thickness measurement.
The data can be displayed in one of several ways.
In Pigs. 10 and 11, there are shown polar graphs of a circumferential portion of casing. In Fig. 10, the casing portion has machined in its northwest ~ circumferential portion an external ~Kr~YKkh~rr groove.
5~ The thickness of the casing wall is shown as a linear function of radius. The azimuthal orientation information ls shown by compass points, designated by N, S, E, W. In the southern circumferential portion of the casing, the wall th~ckness is about 0.36 inches. At the bottom of the groove, the wall thickness is about 0.30 inches. These measurements are very close to the actual wall thicknesses. The scatter from the edges of the groove and in the northeast circumferential portion are caused by surfaces which scatter the acoustic return in directions away from the transducer 31. The confidence level is indicated by the si2e of the data circles. The larger the circles, the higher the confidence level and thus the reliability. ~hus, in the southern circumferential portion, the con~idence level is higher than in the northeastern circumferential portion. Tn Fig. 11, the casing portion has machined in its northwest portion an external flat area. The center of flat area is shown by the cluster of data circles at about 0.22 inches. The edges of the flat area result in scattered data.
The data can also be displayed in a rectangular graph, as shown in Fig. 13. Depth is along the vertical axis and azimuth i8 along the horizontal axis. This type of graph allows the casing wall to be "unwrapped"
for inspection. The data is plotted according to a gray scale where light areas indicate thin walls and d~rker areas indicate thicker wal~s. This portion of casing 'e~
~2~ has a flat area 81 and a~groove 83 ~ ita northwest-Z Zl ~ ~c~tie~ In Fig. 13, the darkest areas indicate areas with no reliable data. Such areas typically occur due to scattering of the acoustic return. scattering occurs where either of the inner or outer sur2aces 21, 23 are not perpendicular to the generated acoustic wave~orm beam. In Fig. 13, the liqhtest area 81 which indicates 12-2l-p~ the bot~om of the groove~a~ juxtaposed with dark areas, t indicatin~ that scattering has occurred at the sides of the flat area. Some scattering is noticeable at the sides o~ the groove 83 as well. The depth scale can be expanded or compressed to a scale which is convenient or wh~ch aids in interpretation. The depth information is provided by the depth instrumentation 68. The azimuthal information is provided by the magnetometer 51. In many cased boreholes, the magnetometer cannot be used because th~ earth's magnetic field i~ attenuated by the casing, ~o a revolution counter iB used instead.
When evaluating casing for corrosion damage, the minimum wall thickness of the corroded area, and the extent and location of the corroded area are the relevant factor~ that are used. The method o~ the present invent~on determines the areas of minimum wall thlckness and provides a measurement of that thickness.
Thus, althouqh data is scattered at the edges of grooves, flats, and other areas of minimum wall thickness, the ~inimu~ wall thickness can be measured and the c~slng can be evaluated. The extent of the corroded araas is provided by the high areal resolution in which full coverage o~ the casing wall is ensured.
The location of the corroded areas is provided by the magnetometer (~or azimuthal information) and depth instrumentation.
The method of the present invention can be used to resolve the thickness of thin walls. Using a transducer having a resonant frequency o~ 2M~z and a bandwidth of 1 - 2.5MHz, a wall thickness of l.S ~m can be resolved.
Uncorroded casinq wall thickness range from lJ4 inches (about 6.5 mm) to 1/2 inches (about 13 mm). The method o~ the pre~ent invention provides a high resolution wall thickness mea~urement, particularly on thin-walled casing, o that a proper evaluation of the condition of the casing can be made.
Although the invention has been described in the context of using off-site processing of the data, processing equipment can be brought on-site to the borehole to allow process~ng. Furthermore, the downhole electronics portion can be modified to allow downhole or real-time processing.
Although thQ method of the pres~nt invention has been described in con~unction with measuring casing wall thickness, other types of tubular goods wall thickness can be measured, such as production tubinq.
Altbouqh this invention has been described with a certain degree of particularity, it is understood that the present disclosure is made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing ~rom the spirit and scope of the invention, reference being had for the latter purpo~e to the appended claims.

Claims (3)

Claims

1. A method for acoustically measuring wall thickness of tubular goods, comprising the steps of:
a) providing acoustical transducer means inside of the tubular goods which are to be investigated, said transducer means having a resonant frequency greater than 1MHz and being directly exposed to fluids in said investigated tubular goods, said transducer means being rotatable so as to scan 360 degrees of the circumference of said investigated tubular goods;
b) exciting said transducer means to generate an acoustic waveform which is directed to a portion of said investigated tubular goods;
c) receiving with said transducer means an acoustic return from said portion of said investigated tubular goods, said acoustic return comprising a reverberation portion, said reverberation portion occurring as a result of said generated acoustic waveform reverberating inside of said portion of said investigated tubular goods;
d) selecting said reverberation portion from said acoustic return;
e) determining the harmonic frequency content of said reverberation portion, determining the frequency difference between two adjacent harmonic frequencies, and determining the wall thickness of said investigated portion from said frequency differences.

2. The method of claim 1 wherein said frequency difference is determined by determining the autocorrelation of the harmonic frequency content.

3. The method of claim 2 wherein said tubular goods comprise casing and said transducer means has a resonant frequency of at least 2MHz.